Hi,
I finished my radiation for pharyngeal (tonsil) cancer last May, and have been unable to swallow since. I take all my nutrition through my PEG tube.
In the interim, I have had about 8 esophageal endoscopies. I'm having another next Tuesday. They are quite easy and should be no problem.
With respect to your mouth not opening wide, my swallow therapist has given me an exercise in which I stack tongue depressors and hold as many as I can between my teeth for a short time. Over time, as your jaw slowly gets pried open, add more tongue depressors. I started a week or so ago with 9 and am up to fourteen. Do it whilst watching TV. It's a snap.
Best of luck, my friend.